1. Technical Field
The present invention relates to a manufacturing method of a polymer composite, in which exfoliated clay nanoparticles are uniformly dispersed, and in particular, to a manufacturing method of an organic modifier-free exfoliated nano clay-polymer composite.
2. Background Art
Unlike conventional composites, a polymer nanocomposite improves physical properties of polymers by introducing piled sheets of layered clay nanoparticles spaced at nanometer intervals, so-called layered silicates. In addition, the conventional composite contains several tens of wt % of inorganic additives used to improve its physical property, whereas the polymer nanocomposite contains inorganic additives below 5 wt %. Furthermore, although the used amount of inorganic additives is reduced, the polymer nanocomposite exhibits its physical property similar to or better than the conventional composite.
Generally, clay nanoparticles used in the polymer nanocomposite are made from layered silicate which has large surface area (about 750 m2/g) and high aspect ratio of 50 or more, and in which a single layer is about 1 nm thick. When the layered silicate is exfoliated, attractive forces applied between layers of clay nanoparticles disappear and the clay nanoparticles are uniformly dispersed in the polymer matrix, a process called as exfoliation. Due to exfoliation, the polymer nanocomposite attains improved physical properties such as mechanical properties, heat-resistance and gas barrier properties, obtained at a lighter weight compared with a conventional composite. The ‘nano clay-polymer composite’ having the above-mentioned advantages, in which clay nanoparticles are uniformly dispersed in a polymer, is applied to products of various purposes, for example functional resins or various coating materials with an enhanced capability for suppressing penetration of gases and liquids, flame prevention, wear resistance and high-temperature stability.
For example, Toyota CRDL group in Japan has developed a nylon nanocomposite by using these advantages, and building up on this, many research groups have applied nylon-6 nanocomposite to timing belt covers and barrier layers for fuel lines of automobiles. In addition, commercial products are continuously developed by using the polymer nanocomposite, for example a thermoplastic olefin (TPO) nanocomposite is applied to automobile step-assist parts and body side moldings by General Motors (GM) of USA, Basell and Southern Clay Products. Honeywell of USA has applied nylon nanocomposites to a barrier material for an intermediate layer of multilayer PET bottle having an improved oxygen barrier property.
Essentially, clay nanoparticles are not uniformly dispersed in a polymer matrix, and thus it is difficult to add the clay nanoparticles during the manufacture of the polymer nanocomposite. That is, clay nanoparticles (layered silicate) serving as fillers in manufacturing a polymer nanocomposite are basically hydrophilic and poorly mixed with the polymer which is generally hydrophobic; thus it is difficult to obtain a polymer nanocomposite having a desired level of improved physical properties. Therefore, to provide hydrophobicity to the layered silicate, it was necessary to substitute an organic modifier having a cationic head and a hydrophobic tail into the space between the silicate layers to create ionic bonds. For example, when an organic modifier having organic ammonium ions is introduced between layers of the clay nanoparticles, due to its structural characteristic, one end of the organic modifier exchanges with such cations as Na+ present between the clay layers and forms bonds to the clay layers. The other end of the organic modifier exhibits a hydrophobic property capable of reacting with or being compatible to the polymer. Further, the spacing between the clay layers is expanded according to the structure of the organic modifier, thereby improving the intercalation capability of a polymer chain in a polymerization or compounding step.
A manufacturing method of an organic clay nanocomposite is described in detail, and first, the clay nanoparticles are dispersed in an excess of deionized water, and the organic modifier to be introduced between the clay layers is treated with HCl (hydrochloric acid) in consideration of the cationic exchange capacity (CEC) of the clay to prepare an organic modifier solution in the form of salt. The organic modifier solution is slowly added while the clay nanoparticle-dispersed solution is agitated, and thus cationic ions in the clay such as Na+ ions are exchanged with the organic modifier, and the organic modifier is intercalated between the clay layers to change the clay nanoparticles in organic clay nanoparticles.
The resultant organic clay nanoparticles are dispersed in various polymers, for example, polyester to manufacture nanocomposites (See U.S. Pat. No. 6,071,988, or 6,084,019). Depending on the manufacturing method of the nanocomposite, nanocomposites containing clay nanoparticles may include intercalation type nanocomposite in which a polymer is intercalated between the clay nanoparticle layers, or an exfoliation type nanocomposite in which clay nanoparticles are completely exfoliated between the nanoparticle layers. Generally, the exfoliation type nanocomposites have better physical properties than the intercalation type nanocomposites.
However, the organic modifier that is ion-bonded to the clay nanoparticles as described above may adversely affect the nanocomposite. In particular, in the case that the nanocomposite is melted at high temperatures according to the purpose of its end use, the color of a product may change due to the thermal decomposition of the organic modifier, or physical, mechanical or optical properties of the nanocomposite may deteriorate as well. As a result, the nancomposite containing the clay nanoparticles may exhibit poorer physical properties than expected.